Butyl rubber is known for its excellent insulating and gas barrier properties. Generally, commercial butyl polymer is prepared in a low temperature cationic polymerization process using Lewis acid-type catalysts, of which a typical example is aluminum trichloride. The process used most extensively employs methyl chloride as the diluent for the reaction mixture and the polymerization is conducted at temperatures on the order of less than −90° C., resulting in production of a polymer in a slurry of the diluent. Alternatively, it is possible to produce the polymer in a diluent which acts as a solvent for the polymer (e.g., hydrocarbons such as pentane, hexane, heptane and the like). The product polymer may be recovered using conventional techniques in the rubber manufacturing industry.
Adhesives (glues) are substances capable of forming and maintaining a bond between two surfaces, and sealants (caulks) are substances used to fill gaps or joints between two materials to prevent the passage of liquids, solids or gases. These two classes of materials are often considered together because quite frequently a given formulation performs the both functions.
Butyl sealants are available as one-component solvent evaporation curing products and as thermoplastic hot melts. There is no curing process, the compound gets its functionality through solvent loss and/or a decrease in temperature. When a sealant is applied, the solvent evaporates or migrates into porous substrates and the tough, rubbery compound is left in place. This is in contrast to other sealant types that cure chemically.
It is known to use a commercial pre-cross-linked butyl rubber such as commercially available Bayer® XL-10000 (or, formerly XL-20 and XL-50) that will add additional strength to the sealant/adhesive. XL-10000 is partially cross-linked with divinylbenzene already in the polymerization stage.
While said commercial pre-cross-linked polymers exhibit excellent properties in many applications, they have a gel content of at least 50 wt. % which sometimes makes the even dispersion of fillers and additives normally used during compounding difficult. This increases the likelihood of inhomogeneous areas within the rubbery article, rendering its physical properties inferior and unpredictable. Also, the Mooney viscosity of this rubber is high, usually 60-70 units (1′+8′@125° C.) which may cause significant processing difficulties, especially in a mixing stage.
Processability-improving polymers are often added to the pre-cross-linked butyl rubber to overcome some of these problems. Such polymers are particularly useful for improving the mixing or kneading property of a rubber composition. They include natural rubbers, synthetic rubbers (for example, IR, BR, SBR, CR, NBR, IIR, EPM, EPDM, acrylic rubber, EVA, urethane rubber, silicone rubber, and fluororubber) and thermoplastic elastomers (for example, of styrene, olefin, vinyl chloride, ester, amide, and urethane series). These processability-improving polymers may be used in the amount of up to 100 parts by weight, preferably up to 50 parts by weight, and most preferably up to 30 parts by weight, per 100 parts by weight of a partially cross-linked butyl rubber. However, the presence of other rubbers dilutes desirable properties of butyl rubber.
RU 2,130,948 discloses the copolymerization of isobutylene with DVB in an aromatic or aliphatic hydrocarbon solvent initiated with a system comprising TiCl4 and triisobutylaluminum. The content of DVB in the monomer feed was 0.1 to 5.0 wt. %, based on isobutylene. The process was carried out in the temperature range −40 to +40° C. The products had low molecular weights (Mv<15,000 g/mol) and were useful as a source for preparing glues. In one example, the process was carried out at +40° C. and the viscosity average molecular weight of the polymer was about 5400 g/mol. This is different from the process described in the present invention, where a typical polymerization temperature was −95° C. and the viscosity average molecular weight of the product was above 200,000 g/mol. The above application did not involve the presence of a chain-transfer agent in the monomer feed during polymerizations.
Co-Pending Canadian Application CA-2,316,741 discloses terpolymers of isobutylene, isoprene, divinyl benzene (DVB) prepared in the presence of a chain-transfer agent, such as diisobutylene, which are substantially gel-free and have an improved processability. However, the above application is silent about applications for sealants and adhesives.
Co-Pending Canadian Application CA 2,386,628 discloses peroxide curable compounds comprising terpolymers of isobutylene, isoprene, divinyl benzene (DVB) prepared in the presence of a chain-transfer agent, such as diisobutylene, which are substantially gel-free and have an improved processability but is also silent about compounds for sealants/adhesives. In the above two applications, an aliphatic conjugated diene, like isoprene, was an integral part of the compounds.